1. Field of the Invention
This invention relates to improvements in a heat pump type air conditioning system for an automotive vehicle in which a passenger compartment can be heated and refrigerated under the action of engine coolant and refrigerant, and more particularly to the improvements to prevent a compressor from its damage upon an excessive rise in discharge pressure.
2. Description of the Prior Art
In recent years, a part of high quality cars and so-called one box-type cars having a relatively large space of a passenger compartment have been equipped with an automotive air conditioning system of a so-called dual air conditioning type including a front unit for air-conditioning a front region (for example, a front seat part) of the passenger compartment and a rear unit for air-conditioning a rear region (for example, a rear seat part) of the passenger compartment. Accordingly, the front and rear regions of the passenger compartment can be independently air-conditioned so as to achieve a comfortable air conditioning for the passenger compartment.
A typical example of the automotive air conditioning systems of such a type is arranged as follows: During the heating operation of the air conditioning system, the front unit uses engine coolant as a heat source while the rear unit uses as a heat source a high temperature and pressure refrigerant which is compressed by a compressor. Such an air conditioning system is arranged to pump up heat from the low temperature outside air in the circulating process or refrigeration cycle of the refrigerant and uses it to heat the passenger compartment. Accordingly, this air conditioning system is called a "heat pump type air conditioning system".
However, drawbacks have encountered in such air conditioning system in case of carrying out the heating operation of the passenger compartment of the automotive vehicle. For example, when the temperature of the outside air is low, for example, in the morning in winter, the temperature of the engine coolant is low at the engine starting, and the temperature rise of the refrigerant is not sharp. Consequently, it is difficult to put the air conditioning system into such a state as to blow warm air into the passenger compartment simultaneously with the starting of operation of the air conditioning system, so that the air conditioning system is low in quick or instant heating ability and is low in heating performance. Particularly in the one-box type cars which are provided with a diesel engine and are large in space of the passenger compartment, the rate of temperature rise of the engine coolant is low as compared with automotive vehicles provided with a usual gasoline engine while it is required to heat the large space, so there is such a tendency as to be low in quick heating ability and heating performance for the passenger compartment.
In view of this, a heat pump type air conditioning system of the following type has now been proposed: The refrigerant is heated under the action of heat of the coolant for a driving system in an electric vehicle, thereby to be increased in enthalpy and becoming high in temperature. Accordingly, the thus heated refrigerant exhibits a high heating ability of the air conditioning system. Such a heat pump type air conditioning system is disclosed in Japanese Patent Provisional Publication No. 7-101227.
It is usual to carry out an on-off control of the compressor in accordance with the discharge pressure of the compressor in order to protect the compressor from its breakage when the discharge pressure of the compressor excessively rises. It will be understood that it is a matter of course that the heat pump type air conditioning system of the above type should employ the on-off control of the compressor to protect the compressor at the excessive rise in discharge pressure of the compressor.
However, in case of protecting the compressor only under the on-off control of the compressor, shock (impact) and noise are generated with the on-off control (for example, the on-off action of a magnetic clutch) of the compressor, thereby lowering the drivability of the vehicle. Additionally, when the compressor is switched OFF, the circulation of the refrigerant is stopped so that the refrigeration cycle does not function. Consequently, for example, during the heating operation of the air conditioning system, cool air is unavoidably blown into the passenger compartment immediately when the compressor is switched OFF, thereby degrading the comfortableness of the air conditioning. Particularly in the above heat pump type air conditioning system, an unusual feeling on the part of the passenger is considerable when the cool air is suddenly blown, on the contrary to the fact that the air conditioning system inherently exhibits a high heating performance. As a result, it is desired to suppress the temperature change of the air blown into the passenger compartment to such a small extent as not to degrade the comfortableness of the passenger.